After the Industrial Revolution, humans established modern society while consuming huge amounts of fossil fuel which increases a carbon dioxide concentration in the air, and the increase of the carbon dioxide concentration is further promoted by environmental destruction such as deforestation or the like. Since global warming is caused by an increase in greenhouse gases such as carbon dioxide, Freon, or methane in the air, it is important to decrease the concentration of carbon dioxide that significantly contributes to the global warming, and various researches into emission regulations or stabilization of carbon dioxide have been conducted around the world.
Among them, a copolymerization reaction of carbon dioxide and epoxide found by Inoue et al. has been expected to be a reaction capable of solving the global warming problem, and research has been actively conducted in view of using carbon dioxide as a carbon source as well as in view of chemical fixation of carbon dioxide. Particularly, a poly(alkylene carbonate) resin formed by polymerization of carbon dioxide and epoxide has been recently spotlighted as a kind of biodegradable resin.
Various catalysts for preparing this poly(alkylene carbonate) resin have been studied and suggested in the past, and as a representative catalyst, a zinc dicarboxylate-based catalyst such as a zinc glutarate catalyst in which zinc and dicarboxylic acid are bonded to each other has been known.
The zinc dicarboxylate-based catalyst as described above, represented by the zinc glutarate catalyst, is formed by reacting a zinc precursor and a dicarboxylic acid such as glutaric acid with each other, and has a fine crystalline particle shape. However, it was difficult to control the zinc dicarboxylate-based catalyst having the crystalline particle shape to have a uniform and fine particle size during a preparation process. The existing zinc dicarboxylate-based catalyst has a particle size of a nanometer scale, but an aggregate having an increased particle size and a decreased surface area is formed in a medium by aggregation of catalyst particles such that at the time of preparing the poly(alkylene carbonate) resin, the activity may be deteriorated.
Meanwhile, according to the X-ray single crystal structure of ZnGA reported by Moonhor Ree et al., the center of each Zn2+ is coordinated to oxygen atoms derived from four different carboxyl groups, and a distance between Zn2+ ions is 4.639 Å (Moonhor Ree et al., Chem. Mater. 2004, 16, 2981).
According to the reported crystal structure of ZnGA, there is almost no void space within the crystal, considering the van der Waals radius. Therefore, a polymerization reaction using the organic zinc catalyst occurs mainly on the surface of the catalyst. For this reason, polymerization activity of the organic zinc catalyst greatly depends on the surface area of the catalyst.
In this regard, it is known that as the zinc precursor used in the preparation of the zinc dicarboxylate-based catalyst has a smaller size, the produced catalyst has higher activity. International Patent Publication No. WO2011/107577 discloses a method of synthesizing a catalyst after increasing a specific surface area of a zinc source used in the preparation of an organic zinc catalyst by surface treatment of the zinc source with organosilane. However, this method requires several steps (reaction, selection, drying, etc.) in the surface treatment of the zinc source with organosilane, and therefore there is a limitation in that this method is inefficient considering improvement degree of the catalytic activity.
For this reaction, there is often a case that the existing known zinc dicarboxylate-based catalyst has a relatively large particle size, a small surface area, and a non-uniform particle shape. Therefore, in the case of performing a polymerization process for preparing a poly(alkylene carbonate) resin using the zinc dicarboxylate-based resin, a sufficient contact area between a reactant and the catalyst is not secured, such that sufficient polymerization activity may not be exhibited. In addition, there is often a case that the activity of the existing zinc dicarboxylate-based catalyst itself is not satisfactory.